Optical attachments for reducing the focal length of an objective lens, also known as focal reducers, are described in the aforementioned U.S. Pre-Grant Pub. No. Number US2013/0064532. The attachments described in US2013/0064532 comprise four lens elements, in order from an object side to an image side: a first lens element having a first negative power and an object-side concave surface; a second lens element having a first positive power and an object-side surface having a curvature CvOb2; a third lens element having a second negative power greater than the first negative power and having an image-side surface having a curvature CvIm3; a fourth lens element having a second positive power and an image-side surface; and wherein the focal reducing attachment has an overall power φ such that (CvOb2+CvIm3)/φ>3 and an overall magnification M such that 0.5<M<1.
Although the focal reducers described in Publication Number US2013/0064532 function well in a variety of applications, they have limitations imposed by their relatively simple structure. Among these limitations are chromatic aberration, field curvature, and high-order coma. These limitations become particularly evident when designing focal reducers having a magnification less than 0.7, where aberrations are found to increase dramatically when an attempt is made to reduce the magnification below about 0.7.